This invention relates generally to optical bistable devices, and, more particularly, to an optical flip-flop system.
Recently much attention has been directed to transmission of speech, data, pictures, or other information by light. More commonly, this type of transmission by optical means is referred to as an optical communication system which is generally characterized as being either radio optical communications, in which the light signals radiate through open space or atmosphere; or guided optical communications, in which the electromagnetic waves constituting the light are guided or regularly refocused along a confined path or optical transmission line. There are numerous advantages in the utilization of light waves instead of radio waves for communication in that the information-carrying capacity of the light waves is substantially greater than that of radio waves and the light radiation can be concentrated into narrower beams or confined in smaller waveguides as a result of the inherent shorter wavelength of the radiation.
It has recently been demonstrated through the transmission of 1.32 .mu.m, 5-psec pulses through a 1 km optical fiber that such transmission can be provided without significant dispersional distortion using short optical pulses and optical fibers. Since such propagating optical signals are not easily influenced by external electromagnetic fields, the utilization of optical communication systems are becomming even more attractive and their potential use in the future appears great. Generally in such optical communication systems, only the transmission of the signals has been optical and the regeneration, modulation, amplification, etc. were performed electrically. It is quite possible, however, that an all optical system would produce substantially better results since it would be substantially simpler, efficient and provide higher speed information processing.
Optical bistability, that is, a new phenomenon of current interest, provides for two different steady-state transmission and or reflection states for the same input intensity. In this new phenomenon, matter and light are closely coupled together and involve phase transitions far from thermal equilibrium. Consequently bistable devices have potential applications as a practical active component for integrated optical circuitry. Therefore, such optical bistable devices are being explored as basic components in all optical signal processing, communication and computing systems.
It has been recognized that intrinsic optical bistable devices have the capability of being switched on with external pulses. However, the switch-off capability generally requires a reduction of the input intensity below the switch-on intensity. Two examples of bistable optical devices which have been used in the past can be found in U.S. Pat. No. 3,813,605, issued on May 28, 1974 to Abraham Szoke and U.S. Pat. No. 3,941,455 issued on Mar. 2, 1976 to John O'Brien. U.S. Pat. No. 3,813,605 relies upon an inconvenient optically controlled switch-off capability, and U.S. Pat. No. 3,941,455 has no optical switch-off capability.
It appears that the first switch-off of an intrinsic optical bistable device by an external optical pulse was reported in an article by S. S. Tarng et al (in which this inventor is one of the co-authors) entitled "External Off and On Switching of a Bistable Optical Device," Applied Physics Letters, 40(3), Feb. 1, 1982, pages 205-207. The above-mentioned article describes a brute force technique in which enough energy is absorbed to heat the device thereby changing the characteristic curves so that the switch-off intensity is above the constant input intensity, resulting in excitonic switch-off. The mechanism is thus the same as for the self-switching of an unstable device undergoing regenerative pulsations. Although such an attempt in switch-off capability is operational it is unlikely to be practical. However, such an attempt appears to be the first step towards active switch-off capability of bistable devices
It would, therefore, be highly desireable if it were possible to provide a bistable optical system (i.e. an optical flip-flop device) in which the source for the incoming beam remains unaffected and yet provides a reliable, extremely fast on-off capability.